1. Field of the Invention
The present invention relates to communication systems, and more particularly, to a communication apparatus for calibrating a tunable compensation filter and method thereof.
2. Description of the Prior Art
Continuing developments of communication technologies have given rise to many communication specifications. One of the most important issues of communication technologies is to develop a solution that supports different communication specifications simultaneously. For instance, Software Defined Radio (SDR) technology supports different communication specifications. A hardware device of the SDR technology is applied to different communication channels and various signal modulations.
Utilizing conventional SDR technology, users can conveniently switch among various communication specifications (e.g., specifications of GSM system and/or Bluetooth, etc.) by using one hardware device (e.g., a mobile phone).
However, because the development of the SDR technology emphasizes both flexible hardware and software a platform, designing an excellent SDR scheme has inherent difficulties.
Usually, there are a tunable filter (e.g., a low pass filter) and a compensation filter for compensating the tunable low pass filter in a conventional SDR communication system, and a 3-dB cut-off frequency of the tunable filter is changeable; in most cases the tunable filter is a sin c like filter. A sin c function is expressed as:sin c(x)=sin(x)/x. 
Please refer to FIG. 1. FIG. 1 is a diagram illustrating frequency responses of a conventional SDR filter. As shown in FIG. 1, a pass band response of the conventional SDR filter is not flat but has a gain drop. The compensation filter is therefore used for compensating the un-ideal frequency response to obtain a wanted flat gain curve of the pass band. Please refer to FIG. 2. FIG. 2 is a diagram illustrating a total frequency response of combining a sin c filter and a 1/sin c filter. The 1/sin c function filter is used as a compensation filter of the sin c filter. As shown in FIG. 2, the compensation filter can be implemented by an ideal 1/sin c filter. Therefore, the required response for the SDR communication device is obtained by multiplying the frequency response of sin c filter with the frequency response of 1/sin c filter, that is, the total frequency response of the two filters will be flat.
Usually, there are a tunable filter (e.g., a low pass filter) and a compensation filter for compensating the tunable low pass filter in a conventional SDR communication system, and a 3-dB cut-off frequency of the tunable filter is changeable; in most cases the tunable filter is a sin c like filter. A sin c function is expressed as:sin c(x)=sin(x)/x. 
Please refer to FIG. 1. FIG. 1 is a diagram illustrating frequency responses of a conventional SDR filter. As shown in FIG. 1, a pass band response of the conventional SDR filter is not flat but has a gain drop. The compensation filter is therefore used for compensating the un-ideal frequency response to obtain a wanted flat gain curve of the pass band. Please refer to FIG. 2. FIG. 2 is a diagram illustrating a total frequency response of combining a sin c filter and a 1/sin c filter. The 1/sin c function filter is used as a compensation filter of the sin c filter. As shown in FIG. 2, the compensation filter can be implemented by an ideal 1/sin c filter. Therefore, the required response for the SDR communication device is obtained by multiplying the frequency response of sin c filter with the frequency response of 1/sin c filter, that is, the total frequency response of the two filters will be flat.
Unfortunately, in real cases the frequency response of the SDR filter is not an ideal sin c function, and the ambient temperature and process deviations also affect the frequency of the sin c filter. Conventionally, the coefficient of the compensation filter is inflexible and thereby fails to compensate the SDR filter successfully even if the frequency response of the SDR filter varies.
In actual circuit design the operation for predicting the frequency response of the SDR filter is both difficult and imprecise. Using the fixed compensation filter (e.g., using a 1/sin c filter) fails to compensate the SDR filter to make the communication device have a pass-band signal which has a flat gain response/gain curve; this leads to a bad signal processing performance at the receiver end.
According to an embodiment of the present invention, a method for adjusting a tunable compensation filter within a communication device is disclosed. The adjusting method includes the following steps: generating at least a detecting signal according to at least a pole of the tunable compensation filter; generating a transmitted signal according to the detecting signal; receiving the transmitted signal via a loop switch; generating a filtered signal by filtering the transmitted signal; generating a received signal by compensating the filtered signal; generating at least an indicating signal by comparing the detecting signal with the received signal; and determining whether or not to adjust at least a coefficient of the tunable compensation filter according to the indicating signal.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.